1. Field of the Invention
This invention relates to a contact type image sensor device which senses an image of objects without reduction of the image. The contact image sensor device is utilized for small-sized facsimile terminal equipment, a bar code reader, and the like.
2. Description of the Related Art
Since a contact image sensor device can sense an image of objects without use of an optical system for reduction of the image, the length of an optical path can be so shortened that equipment comprising the contact image sensor device can be miniaturized. For this reason, recently, contact image sensor devices are widely used as image sensing units in small sized facsimile terminal equipment, bar code readers, and the like.
Though various types of contact image sensor devices are known, the contact image sensor device of the present invention is a type of sensor device which comprises a plurality of photodiodes connected in a matrix and an equal number of blocking diodes provided with each photodiode to prevent a so-called cross talk phenomenon. In this type of contact image sensor device, both the photodiodes and the blocking diodes may be constituted by diodes having the same construction, so that the degree of integration of the sensor elements can be increased and they can be easily fabricated.
U.S. Pat. No. 4,369,372 discloses a photo electro transducer device which comprises n photo electro transducer elements and n diode elements connected respectively to the n photo electro transducer elements. A photo electro transducer element and a respective diode element are formed either on the same side of a common electrode area or formed in layers.
Both the photodiode and the blocking diode have junction capacitances C.sub.p and C.sub.b, respectively. The capacitances C.sub.p and C.sub.b are equivalently connected to the photodiode and the blocking diode, respectively, in parallel. In order to derive a signal current depending on the brightness of a pixel from each photodiode connected in a matrix, a pulsed voltage is applied to the serially connected photodiode and blocking diode. Accordingly, the values of C.sub.p and C.sub.b, especially the ratio of C.sub.p to C.sub.b, is an important factor in determining the dynamic range, magnitude of the signal current and light sensitivity of the image sensor device. However, these points are not described in the U.S. Pat. No. 4,369,372.